1. Field of the Invention
The present invention generally relates to a binder system for use in the field of forming ceramics or ceramic bodies and a method of producing ceramics or ceramic honeycomb bodies utilizing such a binder system. More particularly, the invention is concerned with a binder system that contains a low molecular weight oil-based non-solvent and the use of the binder system in a method for producing ceramic honeycomb bodies.
2. Discussion of the Related Art
Binders and binder systems useful for the manufacture of products from powdered materials, e.g., from particulate ceramic materials, must meet a number of requirements. For example, the binder and binder systems must be compatible with the ceramic material such that a flowable dispersion comprising a relatively high loading of the ceramic material in the binder may be provided. In addition, the "green" preform produced by shaping the dispersion of ceramic powder in the binder should have reasonable strength such that it can be handled. For desirable "burnout" or removal of the binder, the binder should be removable from the shaped ceramic part without incurring distortion or breakage of the part. Furthermore, the binder-free preform should have at least a minimum level of strength, yet be sufficiently free of binder residues that defect-free consolidation is readily achievable.
The formulation of binders meeting these requirements is complex and a large number of different binder formulations have been disclosed in the prior art. Recently, cellulose ether binders have been favored for use in the forming articles of various shapes; i.e., honeycomb substrates. The mixtures are intimately blended and homogeneous and result in the green body having good integrity in size and shape, as well as uniform physical properties. In addition to the binders, these powder mixtures typically include certain organic additives, including for instance, surfactants, lubricants, and dispersants and function as processing aids to enhance wetting thereby producing a uniform batch.
Recently, there has been an increase in the demand for thinner walled, higher cell density cellular structures, complex shaped product, and products having a large frontal area. Thin walled and complex shaped products that are produced, utilizing the current binder technology, i.e., cellulose ether binders, are extremely difficult to handle without causing shape distortion because of the low strength of the "green" preform. One solution/recent trend in the extrusion technology, especially for multicellular honeycomb bodies, comprised of highly filled ceramic powder mixtures, is to extrude a stiffer body without causing proportional increase in pressures. However, attempts to extrude stiffer ceramic batches with the current batch components; i.e., use of the aforementioned cellulose ether binder coupled with the lowering the amount of water and/or including additives such as sodium tallowate or sodium stearate, have been largely unsuccessful because of the higher extrusion pressures resulting from collision of finer particles, and the abrasiveness of the materials involved.
Another attempted solution is to utilize rapid-setting techniques; i.e., solidifying the cell walls of the honeycomb quickly after forming, therefore ensuring that the dimension of the greenware will not be altered in subsequent cutting and handling steps. Prior rapid stiffening methods involve time-delayed stiffening using rapid set waxes as disclosed, for example in U.S. Pat. No. 5,568,652, and/or applying an external field such as an RF field at the die exit. Although these rapid-stiffening methods involve the extrusion of soft batches, which historically, for highly filled ceramic mixtures, have lead to better extrusion quality, these methods have not been overly successful for thin walled cellular structures.
A recent solution disclosed in co-assigned U.S. Pat. App., Ser. No. 60/069,637 (Chalasani et al.) involves a powder mixture, for forming honeycomb structures, that includes inorganic powder materials, binder, solvent for the binder, surfactant, and a component which is non-solvent with respect to the binder, solvent and powder materials. This powder mixture is mixed, plasticized and shaped to form a green ceramic preform body having improved wet green strength and is thus especially suitable for use in the processing of thin walled honeycomb structures. Furthermore, Chalasani discloses a preferred aqueous binder system mixture that includes water, cellulose ether and a hydrophobic non-solvent.
While this Chalasani reference provides significant advances in the capability of the art to form complex, thin-walled ceramic honeycomb bodies through extrusion, the inclusion of this non-solvent in the powder, e.g., light mineral oil, results in additional complications in the "burnout" or removal of the binder. Specifically, it is difficult to remove the binder components from the shaped ceramic part without incurring distortion or breakage of the part. Because of the reduced strength of the thin-wall ceramic honeycomb bodies and the corresponding increase in the dimensional changes during binder removal due to the exothermic nature of the removal of the oil, special considerations in the firing of the ceramic honeycomb must be undertaken to avoid cracking of the ceramic body. Specially designed kilns, apparatus for volatile removal, reduced oxygen containing atmospheres and increased, complicated firing cycles are among the numerous means that have been employed to reduce the differential shrinkage and high cracking frequency experienced during the firing of thin-walled ceramic honeycomb bodies which incorporate the aforementioned binder.
In light of the foregoing inconveniences experienced in the art, there remains a need to develop a binder system which permits a thin-walled ceramic body to be formed and fired into a desired ceramic article without high differential shrinkage and incidences of cracking or defects and which can be quickly and easily removed from the ceramic body.